Negative impedance repeater system



.H. MOURADIAN NEGATIVE IMPEDANCE REPEATER SYSTEM '2 Sheets-Sheet 1 FiledDec. 18, 1934 Fig.1

Fig. 4

.INVENTOR. v I

ATTORNEY.

H. MOURADIAN NEGATIVE IMPEDANCE REPEATER SIYSTEM Nov. 22, 1938.

Filed Dec. 18, 1934 2 Sheets-Sheet 2 [N VEN TOR.

A TTORNEY.

Patented Nov. 22, 1938 UNITED STATES air ta PATENT oFrmE 2,137,696NEGATIVE IMPEDANCE REPEATEnsYsTEM Hughes Mouradian, Philadelphia, Pa. 9Application December 18, 1934, Serial No. 758,056 I 7 Claims. (01.178-44) This invention relates to repeater systems and particularly tothe use of negative impedances as the amplifying and phase correctingelements of such systems.

In an application originally filed on May 3rd, 1927, S. N. 188,497, Iintroduced into the art the conception of a transmission system in whichnegative impedance networks, in 1r or T formation, were introduced atintervals into a transmission line whereby the combination would act asa line of zero loss, viz:

1. The current at the beginning and at the end of the line would havethe same amplitude, regardless of frequency.

2. The phase shift from end to end of the combined system would be zero,regardless of frequency.

This system is illustrated as Fig. 4 of the drawings of said earlierapplication and was reproduced as Fig. 5 of my later continuingapplication S. N. 379,017, which matured into U. S. A. Patent 1,955,681,and subsequently into U. S. A. Patent Re. 19,305, a re-issue of U. S. A.1,955,681. This system made use at each amplifying station of three (3)negative impedances.

In application S. N. 379,017, filed July 17th, 1929, I have shownfurther how the above number of negative impedances required to realizea proper amplifying system could be reduced to two (2). In the case of aT structure it was sufficient to achieve the purpose cited, to providenegative impedances for the two series arms. In the case of a 1rstructure, conversely, it was necessary and sufficient to providenegative ima pedances for the two pillar elements of the 1r structure.

I have discovered that it is possible to further reduce the number ofnegative impedances required to just a single unit. This applicationdiscloses the arrangement which shows the possibility of realizing arepeating system, with full correction for both amplitude and phaseholding true independently of frequency.

This invention will be clearly understood from the followingdescription, which read in conjunction with the attached drawings, ofwhich Fig. 1 shows a simple transmission line translated into a numberof consecutive equivalent 1r structures; Fig. 2 indicatesdiagrammatically the transmission line of Fig. 1 in combination with thenegative impedance 1+ 2) wherein the characters Z1 and Z2 are the sameas shown on Fig. l of the drawings. The impedance i z) 1+ 2) asindicated in this figure, simulating as it does natural transmissionline constants, will usually include a number of distributed elements.This is covered in some detail by Latour in U. S. A.

1,687,253,-aswell as 'by later inventors in this field. Both Z1 and Z2are complex impedances. Actually Z1=Z0 sinh Pl and Z3 Z COth-g' whereinZu=characteristic impedance. P=propagation constant;

Fig. 3 indicates the electrical effect of the bridging of the negativeimpedance be used in conjunction with the arrangementshown on Fig. 3 ofthe drawings. Fig. 5 indicates the impedance changingdevice requiredbetween the terminals of the transmission circuit and the compositetransmission line. Fig. 6 shows the invention with. the structuralarrangement required at intervals along the transmission line.inzassociation with said line. I

It will be noted that the negative impedances required to carry out myinvention into practice must have negative resistance as well asnegative reactance qualities. Electrical devices are already well knownin the art which make it possible to neutralize any complex impedance,including. resistance as well as reactanceelements, and secure suchneutralization independently of frequency. Examples of the types .ofnegative impedance which may be .usecl in conjunction with the presentinvention are shown by Latour, French Patent 501,472, issued in 1920,and U. S. A. Patent 1,687,253. There is already a long list of inventorswho have been granted patents for realizing a negative impedance (-Z1)which is the exact opposite of an impedance (+Z1) such opposition ofphase, holding true for the entire range of useful frequencies in theart of transmitting speech. I

In order to clearly understand the operation of the invention hereindisclosed from an electrical standpoint, reference will be made to Fig.3 of the drawings. It will be clear that the arrangement shown on saidfigure of the drawings represents a series of 1r structures insuccession, each 1r structure consisting of .(1) An architrave impedance(+21). (2) Two pillar impedances (-21)? Now it is wellknown in the artthat the propastructure.

of the drawings.

gation constant (P) of any one 1r structure, shown on Fig. 3 of thedrawings is defined by the relation:-

cosh P=1+ wherein (Z1)is the architrave element and (Z2) is the pillarelement. Applying this well known formula to the case shown on Fig. 3 ofthe drawings, wherein the pillar element is a-negative impedance (Z1) wenote immediately that the hyperbolic cosine of the propagation constant(P) of any one 1r structure is given by It is thus clear that,independently of frequency, cosh P is zero. Now since cosh P is usuallya complex quantity, each term thereof must, of necessity, be separatelyequal to zerotherefore,

(2) cosh a cosh 17:0 (3) sinh a sinh b=0 where (a) and. (b) are thecomponent parts of P=a+7b, (a) representing the attenuation component,(b) the wave length component and (9') representing the usual imaginaryfactor; In order to satisfy relations (2) and (3), we must have and(1:0.

Where the above relations are. satisfied and where they are satisfiedindependently of frequency, it means that the current in traversing Wenow. proceed to determine the impedance characteristics of the systemshown on Fig. 3 This impedance is defined in where (Z1) is thearchitrave' element: and (Z2) is the pillar element. Inthepresentzinstance; we have:

In what follows the negative sign will be used in association with (-jas the. use of this sign results in a positive resistance component forthe characteristic impedance: of the compositetransmission line.

Now, it will be observed that Z1 varies with frequency, since it isequal to Z1=Z0 sinh Pl Unless, therefore, means are provided tocompensate for this variation and secure constancy of the line impedancethere willv be serious reflection effects between the terminatingimpedances at each end of the transmission system indicated on Fig. 3 ofthe drawings. It is thus necessary to provide between the terminals ofthe transmission circuit which. has an. impedance varying in somecomplicated fashion with frequency and the terminating equipment itselfwhich may be assumed to have (as it is usually done) a substantiallyconstant impedance Z0, some intermediate circuit which will have towardsthe transmission line a variable impedance (-j and towards theterminating equipment a constant impedance Z0. A circuit of this type,essentially an impedance changing circuit, is shown on Fig. 5 of thedrawings. It may be noted that this circuit consists of a 1|- structurewhich has towards the transmission line an impedance (9' and towards theterminating equipment an impedance (+Zo). It will be observed that awave arriving from the distant end over the transmission line will finda terminal impedance equal to the characteristic impedance of thetransmission line itself. Thus there will be no reflection effects. Thisresult can be shown to be true, since the parallel impedance of theterminating equipment and the pillar of the 1.- structure having (Zo) asimpedance is infinity, hence all of the energy of the waves arrivingover the transmission line is completely absorbed by the impedance (7the second pillar of the 11- structure. The transmission of energy tothe terminating equipment from the line itself is'thus furnishedlocally. To show more definitely the actual transmission situation underthe conditions as just cited, assume an electromotive force E, actingthrough the line impedance (a' The potential difference at terminals33', 34 of Fig. 5 of the drawings evidently will be E The current (i)actually transmitted to the terminating equipment (+Zo) connected toterminals 3|, 32 is- Canceling the common term (Zo) and simi 33, 34(Fig. 5 of the drawings).

i 2Z Thus, in final analysis, everything happens from end to end of theentire combination, from transmitting station to receiving station atthe other i of the special 1r structure.

. vr structure, is (+20) ohms, of constant value,

even though the impedance of the composite transmission line is variablewith frequency. To'

prove this point, consider first the impedance of the pillar (a' of thespecial 1r structure in parallel with the similar impedance (:i' of thetransmission line. It is obviously equal to This parallel impedance isin series with the architrave impedance n-hi The series impedance of thetwo is therefore:

now this last mentioned impedance is conand (Zo) is exactly (+20). Thusthe condition is obtained wherein there is no impedance irregu- 'laritybetween the terminating equipment and the transmission system, includingunder this designation the special impedance changing device and thecomposite transmission line.

Many other impedance changing arrangements can be conceived which wouldsatisfy the requirements of the problem. But this application is onlyindirectly concerned with these and it is sufficient to show that atleast one circuit is available which it is possible to use to avoid theextremely undesirable impedance irregularities between hue andterminals. The presence of such irregularities might conceivably renderthe line transmission system impractical.

As a conventional proposition, the negative impedance shown on Fig. 4and Fig. 6 of the drawings is that of Latour. As the negative impedancesrequired in the present invention are used in shunt with thetransmission line, it is preferable to use shunt type negativeimpedances from the standpoint of increased stability of operation. Foran understanding of this subject, one of the latest references may beconsulted,- Crisson, U. S. A. 1,776,310, pages 2 and 3, also drawings3-a and 4a.

It may be pointed out that while the negative impedance conceived byLatour is illustrated on the drawings, any other of the many negativeimpedances now known to the art may be used provided their design isproperly correlated with the requirements of the present invention.

Attention is also called to the fact that the impedances and Rassociated with Fig. 4 and with Fig. 6 of the drawings are not indicatedon these drawings in absolute magnitude but are directly proportionalthereto. It will be noted that Mathes has covered this subject in U. S.A. Patent 1,779,382

granted to him. (Page 2, line 86-page 3, line 19.)

I have thus. shown that it is possible to construct a compositetransmission line system which transmits with equal effectiveness allfrequencies with no amplitude distortion and in addition has theadditional invaluable property of transmitting all frequencies with thesame relative phase displacement. Thus all frequencies are transmittedwith exactly the same phase velocity and with no attenuation distortion.

It will be noted that, for purposes of convenience in exposition of theunderlying physical transmission relationships involved, two negative.

impedances are shown on Figure 2 (connected to terminals l5, l6 also I1,l8) and correspondingly two negative impedances (-Z1) are shown onFigure 3 (connected to terminals 5, 6 also 1, 8). Since, however, thetwo negative impedances referred to above are always bridged between thesame electrical points, nothing prevents us from providing a singlenegative impedance device which will be equal to the parallel impedanceof the two negative impedances diagrammatically represented on Figure 2and Figure 3 of the drawings. When the successive sections oftransmission lines provided with negative impedance networks are ofequal length, then clearly the two negative impedances are also of equalmag nitude and hence in that case the single negative impedance requiredis one-half 'the parallel impedance of (Z2) and (Z1"), whichis'specifically covered in claim 1- hereunder. It is clearly obvious,however, that the spacing between negative impedance bridges need not,and in practice probably will not, be the same. Under the last mentionedconditions the magnitude of the single negative impedance bridge will bethat obtained through the application of the well known relationship ofparallel impedances.

In the above disclosure, the transmission line was considered astranslated into a series of equivalent 1r networks. We might just aswell have translated the successive transmission line sections intotheir equivalent T structures. In such a case, proceeding in the manneralready described, we could just as readily construct a transmissionsystem equivalent to that shown on Figure 3 of the drawings by providingseries negative impedances without disturbing the shunt characteristicsof the transmission line. In such a case the diagram corresponding toFigure 3 would consist of a series of T structures each with a seriesarm of (Z2:2) and shunt arm of (Z2:2). The magnitude of the seriesnegative impedance required would be, in such a case, for each seriesarm an impedance equal to (Z1+Z2) :2. The negative impedances of theseries branches of two succeeding T structures can naturally be combinedinto a single series negative impedance. It is important to note thatunder both of the-above two cases, the resulting electrical structurealways consists of networks having successive series and shunt branchesof opposite signs with the shunt branches having exactly one-half themagnitude of the series branches.

I claim:

1. In combination, a transmission line between two terminals withnegative impedances bridged at intervals, the negative impedances ateach bridge being chosen equal to one half the parallel impedance of(Zz) and (Z1), wherein 76 amplitude of voltage and current of the waves.

transmitted over said circuit are concerned and K alsov free fromdistortion from the standpoint of speed of propagation of the variousfrequencies included in the band of frequencies transmitted, whichconsists in negative impedances bridged at substantiallyregularintervals across the trans- 155 mission line, said negativeimpedances having an absolute value proportional to m wherein Z1 is thearchitrave impedance and Z2 the pillar impedance of each section ofnatural line between successive negative impedance bridges.

3. A system of transmission over a composite system, consisting of thecombination of sections of transmission line with bridged negativeimpedance interposed between such sections, the combined systems beingequivalent to a series of 1r structures in consecutive succession, eachsaid 11' structure consisting of an architrave impedmq ance (+Z1) andtwo-pillar impedances each equal to (Z1).

4. A system oftransmission over a composite system consisting ofsuccessive sections of transmission line and negative impedances bridgedbetween said successive line sections, the combined system beingequivalent to a succession of impedance networks with series andparallel branches, in which the parallel branches are of opposite signto the series branches and have one-half the impedance of the seriesbranches in absolute magnitude, substantially as described.

5. A system of transmission over a composite 6 system consisting ofsuccessive sections of transmission line and negative impedancesinserted in series between said successive line sections, the combinedsystem being equivalent to a succession of impedance networks withseries and parallel 10 branches, in which the parallel branches are ofopposite sign to the series branches and have onehalf the impedance ofthe series branches in absolute magnitude, substantially as described.

6. In combination with a transmission line for transmitting electricalwaves of different frequencies, means for effectively compensating forall distortion producedbysaid line in the transmitted waves of allfrequencies, including distortion in phase and amplitude, saidmeanscomprising an electrical structure inserted in shunt with. saidline-said structure in combination with said line having an attenuationconstant equal to zero and a wave length constant equal to 90 betweensuccessive structures.

7. In combination with a transmission line for transmitting electricalwaves of different frequencies and means bridged at intervals toeffectively compensate for all distortion produced by said line, both asregards phase and amplitude, of means located at the terminals of saidtransmission line for preventing the reflection of waves between saidcomposite line and the terminating equipment, said last means beingequally efiective at all frequencies.

HUGHES MOURADIAN.

